The work described here was in part financed by the Swiss Federal Roads Office (research project AGB2012/010). We greatly acknowledge the financial support.

<ol>
	<li>Bertolini, L., Elsener, B., Pedeferri, P., Redaelli, E., Polder, R. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Corrosion of Steel in Concrete: Prevention, Diagnosis, Repair. 2nd edn. , (2013).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Development of an holistic approach to ensure the durability of new concrete construction. , (1997).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Condition+control+and+assessment+of+reinforced+concrete+structures+exposed+to+corrosive+environments.">Condition control and assessment of reinforced concrete structures exposed to corrosive environments.</a> F&#233;d&#233;ration Internationale du B&#233;ton (fib). , (2011).</li><li>Angst, U. 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Res. 39 (12), 1122-1138 (2009).</li><li>Breit, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Zum+Ansatz+eines+kritischen+Chloridgehaltes+bei+Stahlbetonbauwerken.">Zum Ansatz eines kritischen Chloridgehaltes bei Stahlbetonbauwerken.</a> Beton- und Stahlbetonbau. (5), 290-298 (2011).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=European+Standard+EN14629:+Products+and+systems+for+the+protection+and+repair+of+concrete+structures+-+Test+methods+-+Determination+of+chloride+content+in+hardened+concrete.">European Standard EN14629: Products and systems for the protection and repair of concrete structures - Test methods - Determination of chloride content in hardened concrete.</a> European Committee for Standardization. , (2007).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ASTM+C1152+Standard+Test+Method+for+Acid-Soluble+Chloride+in+Mortar+and+Concrete.">ASTM C1152 Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete.</a> ASTM International. , (2012).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=SIA+269/2:2011+Erhaltung+von+Tragwerken+-+Betonbau.">SIA 269/2:2011 Erhaltung von Tragwerken - Betonbau.</a> Schweizerischer Ingenieur- und Architektenverein SIA. , (2011).</li><li>Draft, <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Draft+recommendation+for+repair+strategies+for+concrete+structures+damaged+by+reinforcement+corrosion.">Draft recommendation for repair strategies for concrete structures damaged by reinforcement corrosion.</a> Mater Struct. 27, 415-436 (1994).</li><li>Broomfield, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Corrosion of Steel in Concrete: Understanding, Investigation and Repair. 2nd edn. , (2006).</li><li>Richartz, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Die+Bindung+von+Chlorid+bei+der+Zementerh&#228;rtung.">Die Bindung von Chlorid bei der Zementerh&#228;rtung.</a> Zement-Kalk-Gips. 10, 447-456 (1969).</li><li>Vassie, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reinforcement+corrosion+and+the+durability+of+concrete+bridges.">Reinforcement corrosion and the durability of concrete bridges.</a> Proc. Inst. Civ. Eng. Part 1. 76, 713-723 (1984).</li><li>Angst, U. 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Corros. 63 (12), 1147-1153 (2012).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Standard+Test+Method+for+Corrosion+Potentials+of+Uncoated+Reinforcing+Steel+in+Concrete.">Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete.</a> ASTM International. , (2015).</li><li>Planung, <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=SIA+2006:2013+Planung+Durchf&#252;hrung+und+Interpretation+der+Potenzialmessung+an+Stahlbetonbauten.">SIA 2006:2013 Planung Durchf&#252;hrung und Interpretation der Potenzialmessung an Stahlbetonbauten.</a> Schweizerischer Ingenieur- und Architektenverein SIA. , (2013).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=B3:+Merkblatt+f&#252;r+Elektrochemische+Potentialmessungen+zur+Detektion+von+Bewehrungsstahlkorrosion.">B3: Merkblatt f&#252;r Elektrochemische Potentialmessungen zur Detektion von Bewehrungsstahlkorrosion.</a> Deutsche Gesellschaft f&#252;r Zerst&#246;rungsfreie Pr&#252;fung DGZfP. , (2014).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=B2:+Merkblatt+zur+zerst&#246;rungsfreien+Betondeckungsmessung+und+Bewehrungsortung+an+Stahl-+und+Spannbetonbauteilen.">B2: Merkblatt zur zerst&#246;rungsfreien Betondeckungsmessung und Bewehrungsortung an Stahl- und Spannbetonbauteilen.</a> Deutsche Gesellschaft f&#252;r Zerst&#246;rungsfreie Pr&#252;fung DGZfP. , (2014).</li><li>Soylev, T. A., Fran&#231;ois, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quality+of+steel-concrete+interface+and+corrosion+of+reinforcing+steel.">Quality of steel-concrete interface and corrosion of reinforcing steel.</a> Cem. Concr. Res. 33 (9), 1407-1415 (2003).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=European+Standard+EN+12504-1:2009+-+Testing+concrete+in+structures.+Cored+specimens.+Taking,+examining+and+testing+in+compression.">European Standard EN 12504-1:2009 - Testing concrete in structures. Cored specimens. Taking, examining and testing in compression.</a> European Committee for Standardization. , (2009).</li><li>Andrade, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Test+methods+for+on-site+corrosion+rate+measurement+of+steel+reinforcement+in+concrete+by+means+of+the+polarization+resistance+method.">Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method.</a> Mater Struct. 37 (273), 623-643 (2004).</li><li>Angst, U., Vennesland, &. #. 2. 1. 6. ;., Myrdal, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diffusion+potentials+as+source+of+error+in+electrochemical+measurements+in+concrete.">Diffusion potentials as source of error in electrochemical measurements in concrete.</a> Mater Struct. 42 (3), 365-375 (2009).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=European+Standard+EN+14630:+Products+and+systems+for+the+protection+and+repair+of+concrete+structures+-+Test+methods+-+Determination+of+carbonation+depth+in+hardened+concrete+by+the+phenolphthalein+method.">European Standard EN 14630: Products and systems for the protection and repair of concrete structures - Test methods - Determination of carbonation depth in hardened concrete by the phenolphthalein method.</a> European Committee for Standardization. , (2006).</li><li>Gulikers, J., Andrade, C., Kropp, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Testing+and+Modelling+the+Chloride+Ingress+into+Concrete.">Testing and Modelling the Chloride Ingress into Concrete.</a> RILEM Proceedings PRO 19. , (2000).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ASTM+C1084+Standard+Test+Method+for+Portland-Cement+Content+of+Hardened+Hydraulic-Cement+Concrete.">ASTM C1084 Standard Test Method for Portland-Cement Content of Hardened Hydraulic-Cement Concrete.</a> ASTM International. , (2013).</li><li>Boschmann K&#228;thler, C., Angst, U. M., Wagner, M., Elsener, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Image+analysis+for+determination+of+cement+content+in+concrete+to+improve+accuracy+of+chloride+analyses.">Image analysis for determination of cement content in concrete to improve accuracy of chloride analyses.</a> Cem Concr Res. , (2017).</li><li>Angst, U., Wagner, M., Elsener, B., Leemann, A., Nygaard, P. v. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Method+to+determine+the+critical+chloride+content+of+existing+reinforced+structures.">Method to determine the critical chloride content of existing reinforced structures.</a> VSS report no. 677. , (2016).</li><li>Lambert, P., Page, C. L., Vassie, P. R. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Investigations+of+reinforcement+corrosion:+Part+2+-+Electrochemical+monitoring+of+steel+in+chloride-contaminated+concrete.">Investigations of reinforcement corrosion: Part 2 - Electrochemical monitoring of steel in chloride-contaminated concrete.</a> Mater. Struct. 24 (143), 351-358 (1991).</li><li>Angst, U. M., Elsener, B., Larsen, C. K., Vennesland, &. #. 2. 1. 6. ;. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chloride+induced+reinforcement+corrosion:+Electrochemical+monitoring+of+initiation+stage+and+chloride+threshold+values.">Chloride induced reinforcement corrosion: Electrochemical monitoring of initiation stage and chloride threshold values.</a> Corros. Sci. 53 (4), 1451-1464 (2011).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> RILEM technical committee 235-CTC. , (2015).</li><li>Angst, U., Elsener, B., Larsen, C. K., Vennesland, &. #. 2. 1. 6. ;. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chloride+induced+reinforcement+corrosion:+Rate+limiting+step+of+early+pitting+corrosion.">Chloride induced reinforcement corrosion: Rate limiting step of early pitting corrosion.</a> Electrochim Acta. 56 (17), 5877-5889 (2011).</li><li>Boubitsas, D., Tang, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+influence+of+reinforcement+steel+surface+condition+on+initiation+of+chloride+induced+corrosion.">The influence of reinforcement steel surface condition on initiation of chloride induced corrosion.</a> Mater Struct. 48 (8), 2641-2658 (2015).</li><li>Pacheco, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Corrosion+of+steel+in+cracked+concrete+-+chloride+microanalysis+and+service+life+predictions.">Corrosion of steel in cracked concrete - chloride microanalysis and service life predictions.</a> TU Delft. , (2015).</li><li>Li, L., Sag&#252;&#233;s, A. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chloride+corrosion+threshold+of+reinforcing+steel+in+alkaline+solutions+-+Effect+of+specimen+size.">Chloride corrosion threshold of reinforcing steel in alkaline solutions - Effect of specimen size.</a> Corros. 60 (2), 195-202 (2004).</li><li>Angst, U., R&#248;nnquist, A., Elsener, B., Larsen, C. K., Vennesland, &. #. 2. 1. 6. ;. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probabilistic+considerations+on+the+effect+of+specimen+size+on+the+critical+chloride+content+in+reinforced+concrete.">Probabilistic considerations on the effect of specimen size on the critical chloride content in reinforced concrete.</a> Corros. Sci. 53 (1), 177-187 (2011).</li></ol>

The authors have nothing to disclose.

The most critical steps for the success of the suggested experimental protocol to determine Ccrit are those including the measures taken to prevent false corrosion initiation and other steel bar end effects. In this regard, a variety of approaches were tested, among which the here reported protocol was found to yield the best results28. In further tests, this approach permitted decreasing the rate of false initiation to below 10%. On the one hand, this is owing to coating the border area of the exposed concrete surface with epoxy resin, which increases the length of transport of chlorides through the concrete to the steel bar ends considerably. On the other hand, replacing the original concrete around the steel bar at its ends with a dense, highly alkaline cementitious slurry significantly enhances the corrosion resistance in these areas. Such systems, i.e., coating the steel bar ends with a layer of a polymer-modified cementitious material, have proven successful also in other studies29,30.
Another important aspect is the criterion for corrosion initiation. This criterion is based on RILEM technical committee TC-235 that aimed to recommend a test method for the measurement of Ccrit in samples manufactured in the laboratory31. The rationale is that it is well known that onset of corrosion of unpolarized steel embedded in concrete might take place over a long period of time rather than a well-defined instant30,32. Steel may start corroding at relatively low chloride concentrations but if these are not able to sustain the corrosion process, repassivation will occur, which becomes apparent by a potential increase back to the initial passive level. Such depassivation-repassivation events are typically observed in similar studies30,33,34. The chloride concentration measured at a time of stable corrosion is more relevant for practice than the time at which the very first signs of potential deviations from the passive level become apparent. With the suggested criterion, Ccrit represents the chloride concentration at which corrosion initiates and also stably propagates.
A limitation of the method is that the samples are relatively small, which may have an influence on the results35,36. In order to counteract this, it is suggested to use a relatively high number of samples (ideally 10). The level of confidence depends on the statistical distribution of Ccrit in the actual test area. For more details in this regard, refer to reference36. An additional limitation is that the moisture conditions in the laboratory exposure may differ from those of an actual structure. Finally, the detection of corrosion initiation may be difficult in cases where the potential is generally negative, such as in slag cements or other sulfide containing binders.
To the best of our knowledge, this is the first method of Ccrit determination in engineering structures at a stage prior to corrosion initiation. In contrast to empirical experience from structures, which is by definition obtained after corrosion initiation, this method can be used to measure Ccrit for specific structures or structural members before corrosion degradation occurs; the results can thus be used to assess the risk of (future) corrosion and to predict the remaining time to corrosion initiation (service life modeling). Thus, this method has the potential to be used in material testing, similar to established methods used for mechanical testing (compressive strength, etc.)
The method is currently applied to a number of different concrete infrastructures in Switzerland. This will broaden the severely limited5 knowledge about statistical distributions of Ccrit in structures. Moreover, it will reveal the influence of different factors such as the age of structures, the construction materials used, etc., and thus provide important information for civil engineers and for decision-makers in infrastructure management.

Corrosion of steel in concrete, triggered by the penetration of chlorides through the concrete, is the most frequent cause of the premature degradation of reinforced and pre-stressed concrete structures, and thus presents one of the most important challenges in civil engineering1,2,3,4. Industrialized countries typically have a large inventory of aging concrete infrastructures, built in the second half of the last century, and thus with a history of several decades of exposure to marine climate or deicing salts used on roads. Being able to reliably assess the condition of these structures, i.e., the risk for corrosion, forms the basis for planning maintenance work and for infrastructure management, in general.
The established approach in engineering for managing chloride-induced steel corrosion in concrete is based on a chloride threshold value (also termed critical chloride content, Ccrit)1,5,6. According to this concept, corrosion initiation is thought to occur as soon as the chloride concentration in the concrete at the steel surface exceeds the Ccrit threshold. Thus, assessing the condition of existing structures and estimating the remaining service life typically rely on determining the chloride content at different depths in the concrete, particularly at the depth of the embedded reinforcing steel. A number of reliable and standardized methods exist to measure this chloride concentration in samples of concrete7,8. Comparing the results to Ccrit provides the basis for the assessment of corrosion risk, and planning the type and the extent of repair measures. However, this approach requires knowledge of Ccrit.
Different international standards and recommendations, as well as text books, stipulate values for Ccrit1,3,9,10,11. These are typically around 0.4% chloride by weight of cement, based on long-term experience or early studies12,13. However, it is well known that the actual resistance against Ccrit of a certain structure or structural member is strongly influenced by the materials used, by the age of the structure, and by exposure history and conditions1,5. Thus, it is generally accepted that experience from one structure should only be applied to other structures with caution.
Despite this, it is common engineering practice to use tabulated Ccrit values, independent of the actual structure. This can be explained by the huge scatter of Ccrit in the literature and by the lack of reliable methods to determine Ccrit on-site and in the laboratory5. The approach of using tabulated threshold values in durability assessments is in contrast to structural considerations in condition assessments of aging concrete structures. In the latter case, there exist a number of standardized test methods to determine mechanical properties, such as the strength of the materials in the structure (concrete, reinforcing steel), to be used in the calculations of the structural behavior.
In this work, an experimental protocol to determine Ccrit on samples taken from engineering structures is presented. The approach is based on drilling cores of reinforced concrete in parts of concrete structures where corrosion has not yet initiated. These samples are transferred to the laboratory where they are subjected to an accelerated corrosion test in order to study the conditions for corrosion initiation. The main advantage of the proposed method is that the samples stem from structures and thus exhibit real conditions concerning a number of parameters that are well known to greatly influence Ccrit and which cannot be representatively mimicked in laboratory-produced samples. This includes the type and the age of the concrete (young laboratory concrete versus mature site-produced concrete), the type and surface condition of the reinforcing steel used at the time of construction, and in general the properties of the steel-concrete interface14. Together with the accuracy of laboratory measurement methods, this approach permits the reliable determination of Ccrit for specific structures or structural members.
Application of the suggested protocol in engineering practice will - compared with the common practice of using a constant value for Ccrit - enhance the accuracy of condition assessments and the predictive power of models to analyze the remaining service life. The expected strong increase in repair works of our built infrastructure over the coming decades15 poses an urgent need for such an improvement in engineering of corroding infrastructures.

<table width="651">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="42">
			<td height="42" style="width:216px;height:42px;">Stranded wire</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">cross section at least 0.50 mm&#178;; ideally copper wire, tin plated</td>
		</tr>
		<tr height="63">
			<td height="63" style="width:216px;height:63px;">Self-tapping metal screw</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">any suitable self-tapping screw, typically of length 4-5 mm and diameter around 2.5 mm</td>
		</tr>
		<tr height="42">
			<td height="42" style="width:216px;height:42px;">Ring cable lug</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">suitable to connect screw and cable</td>
		</tr>
		<tr height="63">
			<td height="63" style="width:216px;height:63px;">SikaTop Seal-107</td>
			<td style="width:71px;">Sika</td>
			<td style="width:119px;"></td>
			<td style="width:245px;">two-part polymer modified cementitious waterproof mortar slurry</td>
		</tr>
		<tr height="21">
			<td height="21" style="width:216px;height:21px;">Epoflex 816 L</td>
			<td style="width:71px;">Adisa</td>
			<td style="width:119px;"></td>
			<td style="width:245px;">epoxy coating</td>
		</tr>
		<tr height="63">
			<td height="63" style="width:216px;height:63px;">Exposure tank</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">any suitable tank (e.g. rako box) with a lid;&#160; sufficiently large for exposing the samples</td>
		</tr>
		<tr height="63">
			<td height="63" style="width:216px;height:63px;">Reference electrode</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">Any stable reference electrode suitable for continuous immersion in sodium chloride solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="width:216px;height:21px;">Tap water</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="width:216px;height:21px;">Sodium chloride</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;"></td>
		</tr>
		<tr height="105">
			<td height="105" style="width:216px;height:105px;">Data logger</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">any device able to monitor the potentials of all samples vs. the reference electrode at the specified interval (input impedance &#62;10E7 Ohm)</td>
		</tr>
	</tbody>
</table>

experimental protocol to determine the chloride threshold value for corrosion in samples taken from reinforced concrete structures

The aging of reinforced concrete infrastructure in developed countries imposes an urgent need for methods to reliably assess the condition of these structures. Corrosion of the embedded reinforcing steel is the most frequent cause for degradation. While it is well known that the ability of a structure to withstand corrosion depends strongly on factors such as the materials used or the age, it is common practice to rely on threshold values stipulated in standards or textbooks. These threshold values for corrosion initiation (Ccrit) are independent of the actual properties of a certain structure, which clearly limits the accuracy of condition assessments and service life predictions. The practice of using tabulated values can be traced to the lack of reliable methods to determine Ccrit on-site and in the laboratory.
Here, an experimental protocol to determine Ccrit for individual engineering structures or structural members is presented. A number of reinforced concrete samples are taken from structures and laboratory corrosion testing is performed. The main advantage of this method is that it ensures real conditions concerning parameters that are well known to greatly influence Ccrit, such as the steel-concrete interface, which cannot be representatively mimicked in laboratory-produced samples. At the same time, the accelerated corrosion test in the laboratory permits the reliable determination of Ccrit prior to corrosion initiation on the tested structure; this is a major advantage over all common condition assessment methods that only permit estimating the conditions for corrosion after initiation, i.e., when the structure is already damaged.
The protocol yields the statistical distribution of Ccrit for the tested structure. This serves as a basis for probabilistic prediction models for the remaining time to corrosion, which is needed for maintenance planning. This method can potentially be used in material testing of civil infrastructures, similar to established methods used for mechanical testing.

Figure 7 displays typical steel potentials monitored during chloride exposure in the laboratory. Both examples show that the potential may drop significantly within a very short time, but that the corrosion process may not yet stably propagate, which becomes apparent through an increase of the potential towards its initial passive level. In this protocol, the time of corrosion initiation, i.e., the time at which exposure is stopped and Ccrit is determined, is defined by a marked potential drop followed by 10 days of negative potentials (see section 3.5.2 and the Discussion for more details).
It is common that it may take several months until stable corrosion initiation occurs. This also depends on the initial chloride content already present in the concrete when the samples are taken from the structures. In some cases of the experiments so far conducted, it took more than 1 year until corrosion initiated.
Figure 8 shows an example of Ccrit measured in 11 samples taken from a more than 40 years old road tunnel in the Swiss alps. All these samples were taken from within an area of 1 - 2 m2, thus presumably identically produced and exposed. In this example, the chloride content at the steel surface at the time of sampling was negligible. Additionally, the carbonation front was still far from the steel surface.
Figure 9 shows two examples where the steel potential decreased strongly upon exposure to the chloride-free solution. In one of these specific cases, it was during the subsequent (destructive) examination of the sample found that the concrete at the steel depth was already carbonated. Upon arrival of water at the steel surface, the corrosion process thus immediately started. In the other case, false corrosion initiation occurred at one of the steel bar ends.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56229/56229fig1.jpg" /> 
Figure 1. Schematic Drawing of the Sample Taken from a Structure and Treated in the Laboratory: (a) concrete core with an embedded piece of reinforcing steel; (b) reducing the concrete cover on the exposed side and on the back side by water-cooled diamond cutting; (c) steel bar end protection by means of removing some concrete around the steel and replacing it with a dense cement paste/mortar and subsequent epoxy coating; and (d) epoxy-coating on lateral faces and on end zones of the exposed concrete surface. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56229/56229fig2.jpg" /> 
Figure 2. Schematic Drawing of the Setup for the Corrosion Test in the Laboratory. This shows the placement of the samples in the exposure tank. Spacers are used to ensure contact to exposure solution from the bottom sample surface. All samples are connected to a data logger, measuring the potential of each sample vs. a reference electrode placed in the exposure solution. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56229/56229fig3.jpg" /> 
Figure 3. Schematic Drawing of Possible Time-evolutions of the Steel Potential that Illustrates the Criterion for Corrosion Initiation. At point 1, a potential drop by less than 150 mV from the initial &#34;passive level&#34; occurs; at point 2, a potential drop by at least 150 mV occurs, which is followed by repassivation; at point 3, a potential drop of at least 150 mV occurs (within a max. of 5 days) and the achieved negative potential level is sustained over 10 days. At tini, withdraw the sample from the exposure solution. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56229/56229fig4.jpg" /> 
Figure 4. Schematic Drawing Illustrating the Cutting and Splitting of the Concrete Sample after Detection of Corrosion Initiation. First, a &#34;trench&#34; is cut from the rear side, in parallel to the steel bar. By inserting a chisel or a similar tool, the trench can be used to split the sample as indicated by the arrows. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/56229/56229fig5.jpg" /> 
Figure 5. Photographs Illustrating Sample Analysis After Corrosion Initiation. (a) The two halves of the sample after splitting, and (b) a rust spot is visible at the steel surface after corrosion initiation. Photographs from different samples. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/56229/56229fig6.jpg" /> 
Figure 6. Schematic Drawing Illustrating the Sampling for the Chloride Analysis After Corrosion Initiation: (a) removal of epoxy-coated parts of the split concrete core (purple = cutting planes); (b) removal of the concrete cover down to 2 mm from the steel surface (purple = cutting plane); (c) grinding over a depth interval of +/-2 mm of the steel bar cover depth (red = sampled volume). <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/56229/56229fig7.jpg" /> 
Figure 7. Representative Examples of Measured Potential vs. Time Curves. The typically pronounced potential drops that may be followed by a potential increase (repassivation) until stable corrosion initiation according to the suggested criterion finally initiates. (a) Shows a case where the potential stabilizes on the negative level, and (b) is an example where the potential continues decreasing over the studied period of 10 days. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/56229/56229fig8.jpg" /> 
Figure 8. Example of Ccrit Measured in 11 Samples Taken from Within a Small Concrete Area in a More than 40 Years Old Road Tunnel in the Swiss Alps. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/56229/56229fig9.jpg" /> 
Figure 9. Examples of Marked Potential Decreases Immediately upon Exposure in Chloride-free Solution. In one case, the concrete at the steel depth was already carbonated, thus upon arrival of water at the steel surface, the corrosion process immediately started, leading to a sharp decrease in potential. In the other case, false corrosion initiation occurred at one of the steel bar ends, which here led to a more gradual potential decrease. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig9large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Experimental Protocol to Determine the Chloride Threshold Value for Corrosion in Samples Taken from Reinforced Concrete Structures at JoVE.com

Experimental Protocol to Determine the Chloride Threshold Value for Corrosion in Samples Taken from Reinforced Concrete Structures

We propose a method to measure a parameter that is highly relevant for corrosion assessments or predictions of reinforced concrete structures, with the main advantage of permitting testing of samples from engineering structures. This ensures real conditions at the steel-concrete interface, which are crucial to avoid artifacts of laboratory-made samples.

The aging of reinforced concrete infrastructure in developed countries imposes an urgent need for methods to reliably assess the condition of these ...

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15.1K Views.  ETH Zurich. We propose a method to measure a parameter that is highly relevant for corrosion assessments or predictions of reinforced concrete structures, with the main advantage of permitting testing of samples from engineering structures. This ensures real conditions at the steel-concrete interface, which are crucial to avoid artifacts of laboratory-made samples.